What is Pi?

Engineers use pi all the time for their calculations, but what the heck is pi anyway? Bob and Sparky break it down for us in this video.

Here is the video transcript:

Let’s talk about Pi.

Pi is one of those things in math that comes up again and again. It is used in formulas for the circumference of a circle, the area of a circle, volume of a sphere. I use it to calculate the cutting speed on my lathe. But what is it really?

Well I hope this won’t be too disappointing but it really is a simple thing. It is the ratio of the circumference to the diameter of a circle. That’s it. Here is one way we can see what that ratio is.

First. Let’s measure a circle’s circumference. Remember that the circumference is distance around the circle. Sparky will use a tape measure to measure the distance around this pipe.

Ok. The pipe measures just a little less than 22 inches around. Now, sparky’s tape measure only measures to the nearest 16th of an inch, so let’s keep that in mind. Now let’s compare that to the diameter of the pipe. Sparky will now measure the diameter of the pipe.

The diameter of this pipe is right at 7.0 inches.

We can set these measurements up as a fraction with the circumference on top. This shows that the circumference is bigger than the diameter. How much bigger? Well, just do the division that the fraction is telling us…. see… 22 divided by 7 = 3.14. You see that we can write the division problem different ways, but they all mean the same thing. 22 over 7 is a division problem, we could also write it like this…. or like this….. or like this… we just have to key it into the calculator with the top first, then division key, then the bottom.

So the circumference is a little more than three times bigger than the diameter. This is true of any circle. The Circumference is a little more than 3.14 times bigger than the diameter.

3.14 — does that sound familiar? If you hit the pi button on your calculator you will get number that is very close to 3.14. Here, I will do it on mine……see: 3.14 15 92 65. This number is very close to the actual value of pi, but even it is not exact. Nobody knows the exact value because when you calculate it, the decimal places go on just about forever.

Try this sometime. Measure the circumference and diameter of a piece of pipe, or a wheel (or anything round) and divide those measurements the same way (circumference on top, diameter on bottom). Your result will always be some approximation of pi. If you could measure EXACTLY, your number would be EXACTLY PI. But you can’t. That’s another story for another day.